Progress in the field of scanning probe microscopy SPM has led to the development of new data storage concepts. Specifically, data storage devices such as probe-storage devices have been introduced. These devices have ultra-high storage density and are based on the developments in scanning tunnellng microscopy STM and atomic force microscopy AFM.
An example of a probe-storage device is disclosed in, “The millipede—more than 1,000 tips for future AFM data storage”, by P. Vettiger et al., IBM Journal Research Development, Vol. 44, No. 3, March 2000. This device is based on a thermomechanical principle for storing and/or retrieving information written on a data storage medium. Digital information is stored by using the tip of an AFM cantilever probe having a nanoscale diameter to make indentations of comparable size on the data storage medium. An indentation indicates the presence of information and denotes a binary “1” whereas the absence of an indentation denotes a binary “0”. The indentations are written on multiple tracks that are aligned with a track centre-line TCL and are accessed by scanning the probe with respect thereto. For increased data rate, an array of such probes is used with each probe performing read/write/erase operations over an individual data storage field with an area of, for example, 100 μm by 100 μm.
As mentioned above, the indentations produced on the data storage medium have a comparable size to the tip-diameter of the probe by which they are produced. Typically, an indentation produced in this manner has a conical shape and a diameter of approximately 15 to 30 nm. The accuracy with which the probe positioning, especially when implemented in an array, is conducted with respect to the data storage medium, particularly to access the indentations, plays a role in determining the performance of a data storage device so produced.
It is known to achieve probe-positioning above a data storage medium by using a miniaturised micro-electromechanical system (MEMs) scanner with motion capabilities of, for example, 120 μm in the x and y domains. A control architecture for such a scanner has been disclosed in, “A servo-mechanism for a micro-electro-mechanical-system-based scanning probe storage device” by A. Pantazi et al., Nanotechnology, Vol. 15, pages 612–621, August 2004, in which the x-y positional information of the probes was achieved by thermal position sensors. A closed loop positioning resolution of approximately 2 nm was demonstrated by use of the thermal sensors.
Thermal positioning sensors are a type of global positioning sensor in that their operational capability can span the entire range and/or a sub-range of typical scanners making them effective in providing global positioning information. Global positioning information is information that allows positioning over a range that spans several tracks. Although thermal sensors have a reduced noise component at higher frequencies, for example, ≧50 Hz, this is not the case at lower frequencies, for example, <50 Hz, where ambient temperature variations cause the onset of drift and a noise component that is unsuitable for sensing purposes. Hence, a mechanism for probe-positioning based solely on thermal sensors may not be suitable for long term operation of a probe-storage device.
The specific implementation of multiple probes arranged in an array, where some of the probes and their associated storage fields have been dedicated to the generation of a media-derived position error signal, abbreviated to PES, has been used in a previously-proposed probe positioning technique.
The media-derived PES signal captures deviation of a probe positioning from the track centre-line associated to each data track on a data storage medium. However, although it does not suffer from drift, it has a limited dynamic range, for example, of approximately 50 nm, and a limited operational bandwidth. Putting this in perspective with regard to, for example, a probe storage device where it is desired to position multiple probes with nanometer-scale precision in dedicated data storage fields typically of 100 μm by 100 μm size, a feedback scheme relying solely on a media-derived PES signal may not prove effective.
Accordingly, it is desirable to provide a positioning mechanism for a scanner relative to a data storage medium with global and/or nanometer scale positioning capability and that uses any sensors employed for this purpose more effectively.